In order to evaluate the nature of underground formations surrounding a borehole, it is often desirable to obtain samples of formation fluids from various specific locations in a borehole. The analysis of a fluid sample provides information about the fluid's contents, density, viscosity, bubble point, and other important characteristics. This vital information is used for field planning decisions and for the optimization of upstream and downstream production facilities.
Typically, a fluid sample is obtained by lowering a fluid sampling tool into the well and withdrawing a fluid sample from an underground formation. One example of a sampling tool is the Modular Formation Dynamics Tester (MDT), which is a registered trademark of Schlumberger Technology Corporation, the assignee of this invention. Formation testing tools are disclosed in U.S. Pat. Nos. 4,860,581 and 4,936,139 to Zimmerman et. al, which are assigned to the assignee of the present invention
Hydrocarbon fluids are now seen to be as complex as the rocks in which they accumulate. Compositional gradients, fluid density inversions, compartmentalization and viscosity variations all contribute to the complexities observed today in reservoir fluids. The accurate description of reservoir fluid is as essential as the accurate description of matrix for successful reservoir evaluation and development. DFA is a requirement to uncover these fluid complexities; indiscriminant sampling without DFA is too costly to be performed routinely. DFA allows discovery of the fluid and reservoir complexities, thereby enabling optimal completion, development and production scenarios.
Two fluid characteristics of particular importance are the gas-oil-ratio (GOR) and hydrocarbon compositions. The GOR is the ratio of the volume of the gaseous phase in the native formation fluids over the volume of liquid hydrocarbons at the standard conditions. The GOR is important in designing the upstream and downstream production facilities. For example, if the GOR is high, the surface facilities must be designed to handle a large amount of gas from the well. The measurement of hydrocarbon compositional gradients within a given reservoir compartment or determination of reservoir compartments by identification of differences in fluid compositions and GOR would most likely not occur for other reasons.
The traditional application for MDT fluid analyzers was to monitor filtrate contamination while sampling. After formation fluid is withdrawn from the formation, the fluid passes through a fluid analyzer before it is pumped out of the tool and into the borehole. The fluid analyzer analyzes the sample fluid to determine the level of mud filtrate contamination. Newly developed applications for MDT fluid analyzers as a part of DFA are to measure fluid properties in real-time at reservoir conditions such as GOR, and hydrocarbon compositions together with quantified contamination and cleanup predictions with or without sampling.
One type of fluid analyzer used in a formation testing tool is an optical sensor, which measures the optical density (“OD”) of the sample fluid at several different wavelengths. There are two types of absorption mechanism that contribute to the measured OD of a fluid sample: electron excitation and molecular vibration mode excitation. Absorption by electron excitation occurs when the energy of incident light is transferred to excite delocalized pi electrons to anti-bonding states. This energy level typically corresponds to visible to near-infrared range and gives a shade of color as a result. Molecular vibration absorption is the absorption of a particular frequency of light due to resonance of the chemical bonds in a molecule, and occurs only at specific wavelengths for specific materials. For any given molecule, the wavelength at which vibration absorption occurs is related to the type of chemical bonds and the molecular structure.
One type of such optical sensor is the Optical Fluid Analyzer (“OFA”), which is a trademark of Schlumberger. The OFA, which is a fluid analysis module as found in the MDT mentioned above, determines the identity of the fluids in the MDT flow stream and quantifies the oil and water content. In particular, U.S. Pat. No. 4,994,671 to Safinya, et al. (incorporated herein by reference) describes a borehole apparatus which includes a testing chamber, means for directing a sample of fluid into the chamber, a light source preferably emitting near infrared rays and visible light, a spectral detector, a data base means, and a processing means. Fluids drawn from the formation into the testing chamber are analyzed by directing the light at the fluids, detecting the spectrum of the transmitted and/or backscattered light, and processing the information accordingly (and preferably based on the information in the data base relating to different spectra), in order to quantify the amount of water and oil in the fluid. Thus, the formation oil can be properly analyzed and quantified by type.
Another type of optical sensor is called the Live Fluid Analyzer (“LFA”), which is a trademark of Schlumberger. The LFA has as same capabilities as OFA to quantify the amount of water and oil in the fluid. The LFA is different from the OFA because the LFA includes a methane channel at the wavelength of a “methane peak” and an oil channel at the wavelength of an “oil peak.” A “methane peak” is a molecular vibration absorption peak of methane, whose wavelength corresponds to the resonance of the CH bond in a methane molecule; one methane molecular vibration absorption peak is at a wavelength of about 1,670 nm. The molecular vibration absorption occurs independently of the color of the fluid and independently of whether the methane is in the gas phase or dissolved in the formation fluid. Similarly, an “oil peak” is a molecular vibration absorption peak of oil, whose wavelength corresponds to the resonance of the combination of —CH2 and —CH3 groups in an oil molecule. One oil peak is at a wavelength of about 1,720 nm. GOR can be derived from the ratio of the amplitudes of the “methane peak” and the “oil peak.”
Another type of optical sensor is called the Advanced Fluid Analyzer (“AFA”), which is a trademark of Schlumberger. The AFA has as same capabilities as OFA and LFA to quantify the amount of water and oil in the fluid, which has a U.S. Pat. Publication No. US20040219064 to Raghuraman et al. and is incorporated herein by reference. The AFA also has as same capability as LFA to determine GOR as LFA. The AFA is different from LFA because AFA has upgraded two OD channels in visible regime as “acid channel” and “base channel” to focus on the optical spectra of pH-sensitive dyes in solutions. The acid OD is measured at 445 nm and the base OD is measured at 570 nm. In addition, there is also a reference channel, 815 nm, which has zero response to water and the pH-sensitive dyes. To make a pH measurement, the pH sensitive dye is injected from a dye chamber into the flowline through which the formation fluid is being pumped. The OD ratio between the “acid” and the “base” is then determined from the optical spectra of this dye-water mixture as it flows past the AFA analyzer, further to derive the pH of formation fluid.